How to fix "psql: Could not connect to server"?

Question

PostgreSQL

At some point, the bd stopped responding.
Server - Ubuntu 16.04
DB - PostgreSQL 10
Gives error

"psql: failed to connect to server: No such file or directory. Is
it really running locally and accepting
connections via Unix socket "/var/run/postgresql/.s.PGSQL.5432"?"

the specified folder contains an empty folder "10-main.pg_stat_tmp".
I connect
psql -U postgres -p 5432 -h localhost
either like this
psql
or see the version like this
postgres -V
In the latter case I get

"Command 'postgres' not found"

What suggests that the database crashed, but there are folders with some files with postgres, for example, in /etc/postgresql
and the restart command seems to be successfully executed
sudo systemctl restart postgresql.service
So it didn’t crash? Or crashed? I would look to see if there are data files, but I can't find the location.
Is it treatable and what steps can be taken?